Look at rows and columns. If a numbers only is present in a spesific box we can remove the number from the other rows in the spesific box. Here is a sudoku (which is solved as far as possible with IG, SiSo and SC).

7

8

2

1

9

4

1

4

9

6

2

7

9

2

4

1

1

4

8

5

7

9

6

5

4

5

4

8

3

5

4

4

1

5

7

2

5

4

6

If you look at the solution table you will see that R9C7 and R9C9 are the only cells in R9 which contains the number 1. This means one of these cells must contain the number 1. Therefore we can remove 1 from B9R8 and B9R7. In this case it means R7C7.

56

7

36

8

2

1

9

4

356

58

1

4

35

9

6

2

7

358

9

2

368

357

37

4

36

1

3568

1

4

236

239

8

239

5

2369

7

278

38

9

6

137

5

4

23

13

267

36

5

12379

4

2379

8

2369

1369

3

689

2678

1279

17

2789

16

5

4

4

689

1

39

5

389

7

3689

2

278

5

278

4

6

23789

13

389

1389

The same way you can look at boxes.

If you look in B6 you will see that C9 is the only column where the number 1 exist. Therefore 1 must be in B6C9 and we can remove 1 from B3C9 and B9C9, in this case R9C9.